1. Field of the Invention
The present invention relates, in general, to a diffractive light modulator and, more particularly, to an open hole-based diffractive light modulator, which includes a lower micromirror positioned on a silicon substrate and an upper micromirror provided with open holes spaced apart from the silicon substrate, thus allowing the upper and lower micromirrors to form pixels.
2. Description of the Related Art
Generally, an optical signal processing technology has advantages in that a great amount of data is quickly processed in a parallel manner unlike a conventional digital information processing technology in which it is impossible to process a great amount of data in real time. Studies have been conducted on the design and production of a binary phase filter, an optical logic gate, a light amplifier, an image processing technique, an optical device, and a light modulator using a spatial light modulation theory.
The spatial light modulator is applied to optical memory, optical display device, printer, optical interconnection and hologram fields, and studies have been conducted to develop a display device employing it.
The spatial light modulator is embodied by a reflective deformable grating light modulator 10 as shown in FIG. 1. The modulator 10 is disclosed in U.S. Pat. No. 5,311,360 by Bloom et al. The modulator 10 includes a plurality of reflective deformable ribbons 18, which have reflective surface parts, are suspended on an upper part of a silicon substrate 16, and are spaced apart from each other at regular intervals. An insulating layer 11 is deposited on the silicon substrate 16. Subsequently, a sacrificial silicon dioxide film 12 and a low-stress silicon nitride film 14 are deposited.
The nitride film 14 is patterned by the ribbons 18, and a portion of the silicon dioxide film 12 is etched, thereby maintaining the ribbons 18 on the oxide spacer layer 12 by a nitride frame 20.
In order to modulate light having a single wavelength of λ, the modulator is designed so that thicknesses of the ribbon 18 and oxide spacer 12 are each λ/4.
Limited by a vertical distance (d) between a reflective surface 22 of each ribbon 18 and a reflective surface of the substrate 16, a grating amplitude of the modulator 10 is controlled by applying voltage between the ribbon 18 (the reflective surface 22 of the ribbon 18 acting as a first electrode) and the substrate 16 (a conductive layer 24 formed on a lower side of the substrate 16 to act as a second electrode).
In an undeformed state of the light modulator with no voltage application, the grating amplitude is λ/2 while a total round-trip path difference between light beams reflected from the ribbon and substrate is λ. Thus, a phase of reflected light is reinforced.
Accordingly, in the undeformed state, the modulator 10 acts as a plane mirror when it reflects incident light. In FIG. 2, the reference numeral 20 denotes the incident light reflected by the modulator 10 in the undeformed state.
When proper voltage is applied between the ribbon 18 and substrate 16, the electrostatic force enables the ribbon 18 to move downward toward the surface of the substrate 16. At this time, the grating amplitude is changed to λ/4. The total round-trip path difference is a half of a wavelength, and light reflected from the deformed ribbon 18 and light reflected from the substrate 16 are subjected to destructive interference.
The modulator diffracts incident light 26 using the interference. In FIG. 3, reference numerals 28 and 30 denote light beams diffracted in +/− diffractive modes (D+1, D−1) in the deformed state, respectively.
However, the light modulator by Bloom adopts an electrostatic method to control the position of a micromirror, which is disadvantageous in that operation voltage is relatively high (usually 30 V or so) and the relationship between the applied voltage and displacement is not linear, thus resulting in poor reliability in the control of light.
Meanwhile, the light modulators described in the patents of Bloom can be used as devices for displaying images. In this case, a minimum of two adjacent elements may form a single pixel. Of course, three elements may form a single pixel, or four or six elements may form a single pixel.
However, the light modulators described in the patents of Bloom have a limitation in achieving miniaturization. That is, the light modulators have a limitation in that the widths of the elements thereof cannot be formed to be below 3 μm and the interval between elements cannot be formed to be below 0.5 μm.
Furthermore, a minimum of two elements is required to constitute a diffraction pixel, thus having a limitation in the miniaturization of a device.
In order to solve such problems, a light modulator capable of achieving miniaturization by forming a plurality of protrusions on a micromirror layer is disclosed in Korean Pat. No. P2004-29925 entitled “Hybrid light modulator.”
In the disclosed hybrid light modulator, a plurality of protrusions is provided on the micromirror layer that diffracts incident light by reflecting the incident light. The protrusions are formed in square pillar (bar) shapes, and are arranged to be spaced apart from each other by a regular interval (e.g., the same as the width of the protrusions) along the longitudinal side of the element passing through a recess.
Furthermore, each of the protrusions includes a support the bottom of which is attached to the top surface of the micromirror of the element, and a mirror layer that is formed on the top of the support and adapted to diffract incident light by reflecting the incident light.
In this case, the single mirror layer of one of the protrusions and the portion of the micromirror layer of the element positioned between protrusions form a single pixel.
However, in order to manufacture the hybrid light modulator having such protrusions, a process of separately forming protrusions on the micromirror layer is required, thus incurring additional costs at the time of manufacturing the hybrid light modulator.